In recent years, vigorous development activities have been directed to ceramic metal halide lamps, which are metal halide lamps that employ ceramics as an arc tube material. A ceramic arc tube has advantages in that it allows little reaction with the emission material and provides excellent heat resistance, as compared to a quartz arc tube.
By utilizing the above advantages, it is possible to realize a metal halide lamp which is capable of operating at a higher temperature and provides a higher efficiency and higher color rendition than is possible with quartz.
An example of a metal halide lamp employing a ceramic arc tube is a lamp disclosed in Japanese National Phase PCT Laid-Open Publication No. 2000-511689. This lamp is a metal halide lamp whose ceramic arc tube has enclosed therein not only a halide of at least one of Na (sodium), Tl (thallium), Dy (dysprosium), and Ho (holmium), but also CaI2 (calcium iodide), such that high color rendition with a general color rendering index Ra of 90 or more, as well as white light with a correlated color temperature from 3900K to 4200K, are provided.
However, the metal halide lamp described in Japanese National Phase PCT Laid-Open Publication No. 2000-511689 has an efficiency of about 85 LPW to 90 LPW in the case where the lamp has a power rating (lamp power rating) of 150 W (Watt); thus, it provides a higher efficiency than in the case of employing a quartz tube. Herein, “LPW” is an acronym of “Lumen Per Watt”, with a unit of “lm/W”.
In recent years, from the standpoint of energy saving, there has been a desire for light sources which have a higher efficiency than that of conventional metal halide lamps. While a high-pressure sodium lamp has a very efficiency of about 110 LPW (given a power rating of 180 W), it has a Ra of about 25, indicative of poor color rendition. Therefore, high-pressure sodium lamps are not likely to be used for stores or for high ceilings and the like, but are used for streetlights and the like.
Thus, not only a good lamp efficiency but also high color rendition is vital to illuminations for stores and high ceilings. In general, however, attempts to enhance the efficiency of a light source will result in an increased emission in the green range, for which there exists a strong luminous efficiency, and therefore invite a deterioration of color rendition. In other words, it is supposed to be very difficult to reconcile high efficiency with high color rendition.
The present invention has been made in view of the above problems, and aims to provide a metal halide lamp that exhibits an efficiency (100 LPW or more) which is at least 10% higher than the efficiency (typically 90 LPW) of conventional metal halide lamps, while maintaining high color rendition with a general color rendering index Ra of 70 or more, and preferably 85 or more. A 10% efficiency improvement (increase in luminous flux) is a marginal level for allowing humans to perceive some increase in brightness. The stipulation as to a general color rendering index Ra of 70 or more is believed to ensure high color rendition for enabling distinction of colors of objects in a general working situation at a factory or the like.